1. Field of the Invention
The present invention relates to acoustical structures for absorbing sound waves, and more particularly to acoustical panels comprised principally of a woven fabric and having one or more face portions for transmitting sound waves into a hollow interior of the panel.
2. History of the Prior Art
Acoustical panels of the type which are comprised principally of a woven three-dimensional fabric have found widespread use for many applications. In such panels the fabric is typically woven so as to comprise opposite face plies interwoven with intermediate rib plies. At least one of the face plies is made porous to a desired extent to transmit sound waves into the hollow interior of the panel. Examples of panels of this type are provided by U.S. Pat. No. 3,147,820 of Finger and U.S. Pat. No. 3,700,067 of Dobbs et al.
One problem with panels of this type is that of being able to control the porosity of the woven fabric during manufacture of the panel so as to provide the face plies and in some cases selected ones of the interior rib plies with an optimum degree of porosity upon completion of the panel. As the fabric is impregnated with resin and subjected to elevated temperatures and pressures to cure the resin, the porosity of the fabric is very difficult to control. The individual yarns of the fabric tend to flatten out during curing, often resulting in the resin flowing into and hardening in the spaces between the yarns so as to make the fabric virtually impervious to sound waves. One technique, described in the Dobbs et al patent referred to above, is to use high twist yarns in weaving the fabric. The high twist yarns resist flattening during the curing process and thereby maintain at least some porosity in the acoustical panel. An alternative approach as described in U.S. Pat. No. 3,756,346 of Parker is to serve or braid the yarns prior to weaving the fabric.
For certain applications it may be desirable or essential that the acoustical panel also be provided with adequate structural strength and rigidity. For example such panels may be required to be used in locations where they must not only absorb sound waves but where they are also walked upon or otherwise subjected to substantial loads. Still other panels may be used in underwater applications where they must be capable of absorbing sound waves while at the same time withstanding the severe pressures encountered at lower depths. One arrangement which provides an acoustical panel of substantial strength and rigidity is described in a co-pending application, Ser. No. 290,542, Donald M. Hatch and George D. Lee, Multi-Ply Woven Article Having Acoustical Elements Between Double Plies, filed Sept. 20, 1972 and assigned to the assignee of the present application. In application Ser. No. 290,542, flat, planar acoustical elements of substantial rigidity are inserted between the parallel face plies of the woven fabric. The acoustical elements are provided with a selected number of holes of selected size to maintain optimum porosity.
The various structures referred to above utilize various techniques in providing a woven acoustical panel with both porosity and structural strength. However, it would be desirable to have available alternative techniques for manufacturing such structures, particularly when the difficulties in using many of these prior techniques are considered. In particular it would be desirable to provide the woven fabric with a selected optimum porosity at the end of the manufacturing process in a positive and accurate fashion. At the same time the woven fabric should have sufficient buld and density so as to be readily handled without causing distortion of the acoustic face weave during manufacture of the panel and so as to support and complement the other components of the panel during manufacturing to provide a favorable result.